OMC 120hp WOT fine on land but in the water won't go above 3000 rpm - 18 mph

Hello.

I have an 1972 18 foot Bell Boy with a 120hp omc.

On dry land with the water going through the engine it revs in netural above 4500 rmp. In gear it does the same.

When I put it in the water I can't get it above 3000 and that runs it at about 18 mph.

The engine runs great, fires right up.

I have gone through the troubleshooting tips. The ones I don't know how to do is to determine if the trim is correct. I think I have the right prop.

Any ideas?

Thanks!

Re: OMC 120hp WOT fine on land but in the water won't go above 3000 rpm - 18 mph

Ayuh,.... Welcome Aboard,..... Yer answer is right in Here,....

Don't skip any steps, 'n Assume, Nothing....

Come back with yer results, 'n We'll help ya through it...

Re: OMC 120hp WOT fine on land but in the water won't go above 3000 rpm - 18 mph

Sorry, should have been more specific about where I am in troubleshooting.

Here is where I am....

6. Wrong gear ratio in outdrive - Do not know how to do this one.
8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive - Do not know how to do this
9. Poor cylinder compression Compression Test - Figured this was good because runs fine on dry land
12. Boat overloaded, improperly loaded, or improperly trimmed. - Do not know how to check the trim. Not overloaded for sure

Re: OMC 120hp WOT fine on land but in the water won't go above 3000 rpm - 18 mph

vanvalis said:

Sorry, should have been more specific about where I am in troubleshooting.

Here is where I am....

6. Wrong gear ratio in outdrive - Do not know how to do this one.
8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive - Do not know how to do this
9. Poor cylinder compression Compression Test - Figured this was good because runs fine on dry land
12. Boat overloaded, improperly loaded, or improperly trimmed. - Do not know how to check the trim. Not overloaded for sure

Click to expand...

Ayuh,... I don't think yer drive is trimable...

A Compression Test is Necessary, a motor will rev to the moon with 1/2 it's plug wires off, without a Load...
On the trailer is Irrelevant...

I donno if the Stringer 400 came in more than 1 ratio or not,...
What's the pitch of the prop,..??

Re: OMC 120hp WOT fine on land but in the water won't go above 3000 rpm - 18 mph

Ok, will head out to do a compression test. It has a manual adjustable trim (from what I can see).

Thanks again.

-Van

Re: OMC 120hp WOT fine on land but in the water won't go above 3000 rpm - 18 mph

Oh if you look at the pic of the engine I first posted, you can see the "manual trim". I can adjust that. Could that help me out?

Thanks again!

-Van

Re: OMC 120hp WOT fine on land but in the water won't go above 3000 rpm - 18 mph

Compression test...

140, 140, 145, 142 approx

Re: OMC 120hp WOT fine on land but in the water won't go above 3000 rpm - 18 mph

Looks like a pretty radical pitch on your prop. What are the numbers?

Re: OMC 120hp WOT fine on land but in the water won't go above 3000 rpm - 18 mph

I am pretty new to a lot of this stuff so I need to ask, do you mean the prop numbers?

Thanks

Van

Re: OMC 120hp WOT fine on land but in the water won't go above 3000 rpm - 18 mph

to rev an engine to 4500 rpm on the trailer requires an eye dropper of gasoline
if you remove 1 spark plug wire, it will still rev to 4500
if you remove 2 spark plug wires, it will STILL rev to 4500, demonstrating how little power it requires to rev an unloaded engine to 4500 rpm (probably takes less than 10HP)
So, the trailer testing proved that your engine can make 10HP....not going to propel a runabout with 10HP.
to rev it up under load and make a 18' boat plane, requires 90+HP and a garden hose of gasoline (10 gallons per hour- which is a heavy thick jet stream of ga$oline through the carb). That's why your boat will get 3 or 4 mpg when it is eventually working well. Boats need a ridiculous gas quantity.

also, timing must advance for an engine to make power under load.

compression looks good.

Worry about the things you know how to do first like cleaning the flame arrester screen and check fuel system before moving on to the exotic things like gear ratios and prop pitch.
If it ever worked with that outdrive and prop, those numbers don't change over time.

Re: OMC 120hp WOT fine on land but in the water won't go above 3000 rpm - 18 mph

That all makes sense. As I said before I was an outboard person before so this is all very interesting and great that you are all so helpful. So just for the heck of it my prop is 14x16 - 381186 or 381188 (can't make out the last digit).

Also I adjusted the manual trim so that it is as far away from the transom as I can get it.

A bit of boat history, the original owner had this thing since new, kept it up. sold it to a guy who had it for one year and kind of messed it up. He did mention to me it was not fast enough for water skiing. Of course this didn't bother me being a fishing person. I didn't thing it would be this slow though .

I verified the firing order as well, clean the carb with some simple carb spray.

Will clean the flame arrester screen next. I have 2 fuel filters. The on engine one and an aftermarket one (water seperator). Put a new one of those in few weeks back. The regular filter looks good but will try to get one of those today.

Thanks again.

Van

Re: OMC 120hp WOT fine on land but in the water won't go above 3000 rpm - 18 mph

I would be looking at the advance weights and springs in the distributor first thing once the fuel flow/filters etc is verified (and don't overlook fuel quality/water, and the anti siphon valve at the tank pickup fitting.

Re: OMC 120hp WOT fine on land but in the water won't go above 3000 rpm - 18 mph

Well made some adjustments to the throttle and the trim and adjusted points. Went out on the lake, got up to 22 mph and then it started to overheat really bad. Lots of smoke coming from the water exhaust port on the side of the boat (comes out of the engine and has a closable valve on it. I had it open. Fresh water cooling system was full. Engine got real loud and stayed that way. Let her cool down, headed back in slow. Was running hot but not super bad. Engine was still really loud.

I wonder if I destroyed the engine?

Felt like scuttling the thing.

The saga continues.

-Van